The scanning electron microscope was used to investigate the morphological characteristics of the smeared layer on the walls of root canals that had been instrumented with K-type files and irrigated with a 5.25% solution of NaOCl. The smeared material was examined from two perspectives. First, the scanning electron microscope was used to "look down onto" the smeared layer covering the canal wall. Second, the smeared material was looked at from the side or profile view. This was done by using the scanning electron microscope to scan the junction between the fractured dentinal tubules and the surface of the canal wall. The smeared material was found to consist of two confluent components; the smeared layer on the surface of the canal wall and the smeared material which had been packed into some of the dentinal tubules. The smeared layer on the canal wall was typically about 1- to 2-μm thick. The depth of the tubular packing varied from a few micrometers up to 40 μm.
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